Certain spiropyrans are known to exhibit photochromic properties; that is, they undergo reversible, light-induced photochemical processes that result in a color change. The process attributed to the photochromism of spiropyrans is generalized by the following equation (1): ##STR1## when the reversibility indicated in equation (1) is disrupted by chemical reaction, the phenomenon of photochromism no longer operates and the spiropyran is fixed in its colored form. The ability to fix the colored form of photochromic spiropyrans has led to their use in, for example, optical data storage. (See S. Maslowski, "High Density Data Storage UV Sensitive Tape", Applied Optics, 13, No. 4, 857 (1974).
Several chemical methods for fixing spiropyrans in their colored form are known. See, for example, G. Brown, "Photochromism, Techniques of Chemistry Vol. III", Wiley-Interscience 1971 (New York), p 268. One well known method involves the use of a hydrogen halide, HX, which is generated by light within a polymeric substrate containing the spiropyran. Precursors for HX include organic compounds containing bromine, which compounds absorb light and generate hydrogen bromide which reacts with the spiropyran to give a colored salt as generalized in equation (2): ##STR2##
The system of equation (2) has several inherent disadvantages: the HX precursors, e.g., 2,2,2-tribromoethanol, are unstable to sunlight and moisture; also, the HX acid formed is volatile and the photoinduced formation of the HX acid is not necessarily rapid or complete, i.e., the reaction can continue after irradiation is terminated. The present invention helps to solve these problems by providing a novel process for fixing spiropryans in their colored form.